Enzymatic description of the anhydrofructose pathway of glycogen degradation; I. Identification and purification of anhydrofructose dehydratase, ascopyrone tautomerase and alpha-1,4-glucan lyase in the fungus Anthracobia melaloma

Molecular Basis of Absorption at 340 nm of 3-Ketoglucosides under Alkaline Conditions

Transient absorption at 340 nm under alkaline conditions has long been used to detect the presence of 3-keto-O-glycosides without understanding the molecular basis of the absorbance. The time course of A340 nm for the alkaline treatment of 3-ketolevoglucosan, an intramolecular 3-keto-O-glycoside, was investigated to identify the three products generated through alkaline treatment. By comparing the spectra of these compounds under neutral and alkaline conditions, we identified 1,5-anhydro-D-erythro-hex-1-en-3-ulose (2-hydroxy-3-keto-D-glucal) as being the compound responsible for the absorption.

Enzymatic Synthesis of 1,5-Anhydro-4-O-β-D-glucopyranosyl-D-fructose Using Cellobiose Phosphorylase and Its Spontaneous Decomposition via β-Elimination

Cellobiose phosphorylase from Cellvibrio gilvus was used to prepare 1,5-anhydro-4-O-β-D-glucopyranosyl-D-fructose [βGlc(1→4)AF] from 1,5-anhydro-D-fructose and α-D-glucose 1-phosphate. βGlc(1→4)AF decomposed into D-glucose and ascopyrone T via β-elimination. Higher pH and temperature caused faster decomposition. However, decomposition proceeded significantly even under mild conditions. For instance, the half-life of βGlc(1→4)AF was 17 h at 30 °C and pH 7.0. Because βGlc(1→4)AF is a mimic of cellulose, in which the C2 hydroxyl group is oxidized, such decomposition may occur in oxidized cellulose in nature. Here we propose a possible oxidizing pathway by which this occurs.

α-Glucosidases and α-1,4-glucan lyases: structures, functions, and physiological actions

α-Glucosidases (AGases) and α-1,4-glucan lyases (GLases) catalyze the degradation of α-glucosidic linkages at the non-reducing ends of substrates to release α-glucose and anhydrofructose, respectively. The AGases belong to glycoside hydrolase (GH) families 13 and 31, and the GLases belong to GH31 and share the same structural fold with GH31 AGases. GH13 and GH31 AGases show diverse functions upon the hydrolysis of substrates, having linkage specificities and size preferences, as well as upon transglucosylation, forming specific α-glucosidic linkages. The crystal structures of both enzymes were determined using free and ligand-bound forms, which enabled us to understand the important structural elements responsible for the diverse functions. A series of mutational approaches revealed features of the structural elements. In particular, amino-acid residues in plus subsites are of significance, because they regulate transglucosylation, which is used in the production of industrially valuable oligosaccharides. The recently solved three-dimensional structure of GLase from red seaweed revealed the amino-acid residues essential for lyase activity and the strict recognition of the α-(1 → 4)-glucosidic substrate linkage. The former was introduced to the GH31 AGase, and the resultant mutant displayed GLase activity. GH13 and GH31 AGases hydrate anhydrofructose to produce glucose, suggesting that AGases are involved in the catabolic pathway used to salvage unutilized anhydrofructose.

A New Perspective on Sustainable Soil Remediation-Case Study Suggests Novel Fungal Genera Could Facilitate in situ Biodegradation of Hazardous Contaminants

Deciding upon a cost effective and sustainable method to address soil pollution is a challenge for many remedial project managers. High pressure to quickly achieve cleanup goals pushes for energy-intensive remedies that rapidly address the contaminants of concern with established technologies, often leaving little room for research and development especially for slower treatment technologies, such as bioremediation, for the more heavily polluted sites. In the present case study, new genomic approaches have been leveraged to assess fungal biostimulation potential in soils polluted with particularly persistent hydrophobic contaminants. This new approach provides insights into the genetic functions available at a given site in a way never before possible. In particular, this article presents a case study where next generation sequencing (NGS) has been used to categorize fungi in soils from the Atlantic Wood Industries Superfund site in Portsmouth, Virginia. Data suggest that original attempts to harness fungi for bioremediation may have focused on fungal genera poorly suited to survive under heavily polluted site conditions, and that more targeted approaches relying on native indigenous fungi which are better equipped to survive under site specific conditions may be more appropriate.

The structure of substrate-free 1,5-anhydro-D-fructose reductase from Sinorhizobium meliloti 1021 reveals an open enzyme conformation

1,5-Anhydro-D-fructose (1,5-AF) is an interesting building block for enantioselective and stereoselective organic synthesis. Enzymes acting on this compound are potential targets for structure-based protein/enzyme design to extend the repertoire of catalytic modifications of this and related building blocks. Recombinant 1,5-anhydro-D-fructose reductase (AFR) from Sinorhizobium meliloti 1021 was produced in Escherichia coli, purified using a fused 6×His affinity tag and crystallized in complex with the cofactor NADP(H) using the hanging-drop technique. Its structure was determined to 1.93 Å resolution using molecular replacement. The structure displays an empty substrate-binding site and can be interpreted as an open conformation reflecting the enzyme state shortly after the release of product, presumably with bound oxidized cofactor NADP⁺. Docking simulations indicated that amino-acid residues Lys94, His151, Trp162, Arg163, Asp176 and His180 are involved in substrate binding, catalysis or product release. The side chain of Lys94 seems to have the ability to function as a molecular switch. The crystal structure helps to characterize the interface relevant for dimer formation as observed in solution. The crystal structure is compared with the structure of the homologue from S. morelense, which was solved in a closed conformation and for which dimer formation in solution could not be verified but seems to be likely based on the presented studies of S. meliloti AFR.

Inhibition spectrum studies of microthecin and other anhydrofructose derivatives using selected strains of Gram-positive and -negative bacteria, yeasts and moulds, and investigation of the cytotoxicity of microthecin to malignant blood cell lines

AIMS

To prepare 1,5-anhydro-d-fructose (AF) derivatives, test their microbial inhibition spectrum, and to further examine the most effective AF derivative against Pseudomonas aeruginosa and malignant blood cell lines.

METHODS AND RESULTS

Microthecin and nine other AF derivatives were synthesized from AF. The 10 compounds were tested in vitro against Gram-positive (GP) and Gram-negative (GN) bacteria, yeasts and moulds using a well diffusion method and in a Bioscreen growth analyser. Of the test compounds, microthecin exhibited the most significant antibacterial activity at 100-2000 ppm against both GP and GN bacteria, including Ps. aeruginosa. Further tests with three malignant blood cell lines (Mutu, Ramos, Raji) and one normal cell line indicated that microthecin was a cell toxin, with a cell mortality >85% at 50 ppm. The other nine AF derivatives demonstrated low or no antimicrobial activity.

CONCLUSIONS

Microthecin was active 100-2000 ppm against GP and GN bacteria including Ps. aeruginosa, but was inactive against yeasts and moulds. Microthecin was also a cytotoxin to some mammalian cell lines.

SIGNIFICANCE AND IMPACT OF THE STUDY

Microthecin might have potential for development as a novel drug against Ps. aeruginosa and to target cancer cells. It might also be developed as a food processing aid to control bacterial growth.

Biosynthesis of ethers: unusual or common natural events?

Ether bonds are found in a wide variety of natural products--mainly secondary metabolites--including lipids, oxiranes, terpenoids, flavonoids, polyketides, and carbohydrate derivatives, to name some representative examples. To furnish such a biodiversity of structures, a large number of different enzymes are involved in several different biosynthetic pathways. Depending on the compound and on the (micro) environment in which the reaction is performed, ethers are produced by very different (enzymatic) reactions, thus providing an impressive display of how Nature has combined evolution and thermodynamics to be able to produce a vast number of compounds. In addition, many of these compounds possess different biological activities of pharmacological interest. Moreover, some of these ethers (i.e., epoxides) have high chemical reactivity, and can be useful starting materials for further synthetic processes. This review aims to provide an overview of the different strategies that are found in Nature for the formation of these "bioethers". Both fundamental and practical insights of the biosynthetic processes will be discussed.

The anhydrofructose pathway and its possible role in stress response and signaling

Anhydrofructose (AF) pathway describes the catabolism of alpha-1,4-glucans of glycogen, starch and maltosaccharides to various metabolites via the central intermediate AF. The reaction sequence of the pathway consists of more than 10 enzymatic steps. This pathway occurs in certain bacteria, fungi, algae and mammals. In this communication, the AF pathway and its regulatory mechanisms in these organisms are presented and the metabolites of this pathway as antioxidants and antimicrobials in biotic and abiotic stress responses and in carbon starvation signaling are discussed.

A new chemical synthesis of Ascopyrone P from 1,5-anhydro-d-fructose

The naturally occurring antioxidant Ascopyrone P (1,5-anhydro-4-deoxy-D-glycero-hex-1-en-3-ulose, 1) was prepared from the rare sugar 1,5-anhydro-D-fructose (AF, 3) in three steps in an overall yield of 36%. Thus, acetylation of 3 afforded the enolone 3,6-di-O-acetyl-1,5-anhydro-4-deoxy-D-glycero-hex-3-en-2-ulopyranose (4), which could be isomerised to 2,6-di-O-acetyl-1,5-anhydro-4-deoxy-D-glycero-hex-1-ene-3-ulose (6). Deacetylation of 6 under mild conditions gave crystalline Ascopyrone P (1).

Catabolism of 1,5-anhydro-D-fructose in Sinorhizobium morelense S-30.7.5: discovery, characterization, and overexpression of a new 1,5-anhydro-D-fructose reductase and its application in sugar analysis and rare sugar synthesis

The bacterium Sinorhizobium morelense S-30.7.5 was isolated by a microbial screening using the sugar 1,5-anhydro-D-fructose (AF) as the sole carbon source. This strain metabolized AF by a novel pathway involving its reduction to 1,5-anhydro-D-mannitol (AM) and the further conversion of AM to D-mannose by C-1 oxygenation. Growth studies showed that the AF metabolizing capability is not confined to S. morelense S-30.7.5 but is a more common feature among the Rhizobiaceae. The AF reducing enzyme was purified and characterized as a new NADPH-dependent monomeric reductase (AFR, EC 1.1.1.-) of 35.1 kDa. It catalyzed the stereoselective reduction of AF to AM and also the conversion of a number of 2-keto aldoses (osones) to the corresponding manno-configurated aldoses. In contrast, common aldoses and ketoses, as well as nonsugar aldehydes and ketones, were not reduced. A database search using the N-terminal AFR sequence retrieved a putative 35-kDa oxidoreductase encoded by the open reading frame Smc04400 localized on the chromosome of Sinorhizobium meliloti 1021. Based on sequence information for this locus, the afr gene was cloned from S. morelense S-30.7.5 and overexpressed in Escherichia coli. In addition to the oxidoreductase of S. meliloti 1021, AFR showed high sequence similarities to putative oxidoreductases of Mesorhizobium loti, Brucella suis, and B. melitensis but not to any oxidoreductase with known functions. AFR could be assigned to the GFO/IDH/MocA family on the basis of highly conserved common structural features. His6-tagged AFR was used to demonstrate the utility of this enzyme for AF analysis and synthesis of AM, as well as related derivatives.

A 90-day toxicological evaluation of 1,5-anhydro-d-fructose in Sprague-Dawley rats

1,5-anhydro-d-fructose (1,5-AF) is a novel monosaccharide produced by the action of alpha-1,4-glucan lyase (EC 4.2.2.13) on glycogen, starch, or related substrates such as maltose and maltosaccharides. 1,5-AF is of interest as a compound to be used as a food supplement because of its antioxidant, antimicrobial, and antidiabetic properties. This enforces the safety of 1,5-AF and therefore, in the current study, four groups of male and female Sprague-Dawley rats were provided with 1,5-AF in the drinking water (at 0 or 1.0 g/kg body weight daily) for a period of 90 days (n=10 in each group). All the animals survived, and no clinical signs of toxicity or alterations in hematological or clinical chemistry parameters were observed. Furthermore, organ weight and histopathological examination of brain, heart, urinary bladder, gastrointestinal tract, and pancreas were normal after 1,5-AF treatment. Moreover, there was no change in food consumption, water intake, or body weight gain in rats receiving 1,5-AF. In conclusion, administration of 1,5-AF did not induce any significant toxicological effects and, therefore, 1,5-AF seems safe to administer in vivo over a long period of time.

Enzymatic description of the anhydrofructose pathway of glycogen degradation

The anhydrofructose pathway describes the degradation of glycogen and starch to metabolites via 1,5-anhydro-D-fructose (1,5AnFru). Enzymes that form 1,5AnFru, ascopyrone P (APP), and ascopyrone M (APM) have been reported from our laboratory earlier. In the present study, APM formed from 1,5AnFru was found to be the intermediate to the antimicrobial microthecin. The microthecin forming enzyme from the fungus Phanerochaete chrysosporium proved to be aldos-2-ulose dehydratase (AUDH, EC 4.2.1.-), which was purified and characterized for its enzymatic and catalytic properties. The purified AUDH showing a molecular mass of 97.4 kDa on SDS-PAGE was partially sequenced. Total 332 amino acid residues in length were obtained, representing some 37% of the AUDH protein. The obtained amino acid sequences showed no homology to known proteins but to an unannotated DNA sequence in Scaffold 62 of the published genome of the fungus. The alignment revealed three introns of the identified AUDH gene (Audh; ph.chr), thus the first gene coding for a neutral sugar dehydratase is identified. AUDH was found to be a bi-functional enzyme, being able to dehydrate 1,5AnFru to APM and further isomerizing the APM formed to microthecin. The optimal pH for the formation of APM and microthecin was pH 5.8 and 6.8, respectively. AUDH showed 5 fold higher activity toward 1,5AnFru than toward its analogue glucosone, when tested at concentrations from 0.6 mM to 0.2 M. Based on the characteristic UV absorbance of microthecin (230 nm) and APM (262 nm) assay methods were developed for the microthecin forming enzymes.

A novel family of glucosyl 1,5-anhydro-d-fructose derivatives synthesised by transglucosylation with dextransucrase from Leuconostoc mesenteroides NRRL B-512F

1,5-Anhydro-d-fructose (AF), a metabolite of starch/glycogen degradation, is a good antioxidant. With the prospect of increasing its applications and use as a food ingredient, AF glucosylation catalysed by the dextransucrase from Leuconostoc mesenteroides NRRL B-512F was performed in the presence of sucrose. This led to AF glucosylated derivatives containing alpha-(1-->6) linkages named 1,5-anhydro-d-fructo-glucooligosaccharides (AFGOS). LC-MS analyses showed that AFGOS with a degree of polymerisation (DP) of up to 7 were synthesised. The amount of AFGOS produced and the average DP increased by using a high sucrose/AF molar ratio and high total sugar concentration. AFGOS were proved to present antioxidant properties quite similar to AF.

Basic toxicology and metabolism studies of 1,5-anhydro-d-fructose using bacteria, cultured mammalian cells, and rodents

1,5-Anhydro-D-fructose (AF) is a monosaccharide occurring in edible morels, red seaweeds and certain mammalian tissues. It can be formed directly from starch and glycogen in vivo by alpha-1,4-glucan lyase (EC 4.2.2.13). In this study, the toxicity, absorption and metabolism of AF using bacteria, mammalian cells, rat and mouse models were examined. In Ames test, AF showed no genotoxicity using five strains of the bacterium Salmonella typhimurium TA 98, 100, 102, 1535 and 1537. AF caused no mammalian gene mutation as tested with mouse lymphoma L5178Y cells. AF did not cause toxic symptoms in rats when it was administered as a single oral dose of 5 g/kg and observed over a 14-day period. Furthermore, at necropsy, no signs of abnormality were detected. Daily intraperitoneal (ip) administration of 2 g/kg AF to mice did not induce adverse effects throughout a 28-day period. Radioactive tracing experiments using 14C-labeled AF indicated that AF was efficiently absorbed since the major portion of radioactive material was recovered in urine. Further work using unlabeled AF indicated that the cyclic polyol 1,5-anhydro-D-sorbitol (AS) increased dramatically in both blood and urine upon AF administration at 1 g/kg ip, suggesting the existence of an efficient reduction mechanism from AF to AS, which was then excreted in urine. In conclusion, these studies indicate that AF had low or no toxicity and showed no mutagenicity.